Cathode materials for secondary (rechargeable) lithium batteries

ABSTRACT

The invention relates to materials for use as electrodes in an alkali-ion secondary (rechargeable) battery, particularly a lithium-ion battery. The invention provides transition-metal compounds having the ordered-olivine or the rhombohedral NASICON structure and the polyanion (PO 4 ) 3−  as at least one constituent for use as electrode material for alkali-ion rechargeable batteries.

This is a continuation of application Ser. No. 14/313,446 filed Jun. 24,2014; which is a continuation of application Ser. No. 13/645,341 filedOct. 4, 2012; which is a continuation of application Ser. No.13/269,299, filed Oct. 7, 2011, now U.S. Pat. No. 8,282,691 Issued Oct.9, 2012; which is a continuation of application Ser. No. 12/952,978,filed Nov. 23, 2010, now U.S. Pat. No. 8,067,117 issued Nov. 29, 2011;which is a continuation application of application Ser. No. 11/179,617,filed Jul. 13, 2005, now abandoned; which is a continuation ofapplication Ser. No. 10/902,142, filed Jul. 30, 2004, now abandoned;which is a continuation of application Ser. No. 10/307,346, filed Dec.2, 2002, now abandoned; which is a continuation of application Ser. No.08/998,264, filed Dec. 24, 1997, now U.S. Pat. No. 6,514,640 issued Feb.4, 2003; which is a continuation-in-part of application Ser. No.08/840,523 filed Apr. 21, 1997, now U.S. Pat. No. 5,910,382 issued Jun.8, 1999. U.S. patent application Ser. No. 08/840,523 also claimspriority to provisional patent Application No. 60/032,346, filed Dec. 4,1996, and provisional patent Application No. 60/016,060, filed Apr. 23,1996. The entire text of each of the above-referenced disclosures isspecifically incorporated by reference herein without disclaimer. TheRobert A. Welch Foundation, Houston, Tex., supported research related tothe present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to secondary (rechargeable) alkali-ionbatteries. More specifically, the invention relates to materials for useas electrodes for an alkali-ion battery. The invention providestransition-metal compounds having the ordered olivine, the modifiedolivine or the rhombohedral NASICON structure and containing thepolyanion (PO₄)³⁻ as at least one constituent for use as an electrodematerial for alkali-ion rechargeable batteries.

2. Description of the Related Art

Present-day lithium batteries use a solid reductant as the anode and asolid oxidant as the cathode. On discharge, the metallic anode suppliesLi⁺ ions to the Li⁺-ion electrolyte and electrons to the externalcircuit. The cathode is typically an electronically conducting host intowhich Li⁺ ions are inserted reversibly from the electrolyte as a guestspecies and charge-compensated by electrons from the external circuit.The chemical reactions at the anode and cathode of a lithium secondarybattery must be reversible. On charge, removal of electrons from thecathode by an external field releases Li⁺ ions back to the electrolyteto restore the parent host structure, and the addition of electrons tothe anode by the external field attracts charge-compensating Li⁺ ionsback into the anode to restore it to its original composition.

Present-day rechargeable lithium-ion batteries use a coke material intowhich lithium is inserted reversibly as the anode and a layered orframework transition-metal oxide is used as the cathode host material(Nishi et al., U.S. Pat. No. 4,959,281). Layered oxides using Co and/orNi are expensive and may degrade due to the incorporation of unwantedspecies from the electrolyte. Oxides such as Li_(1±x)[Mn₂]O₄, which hasthe [M₂]O₄ spinel framework, provide strong bonding in three dimensionsand an interconnected interstitial space for lithium insertion. However,the small size of the O²⁻ ion restricts the free volume available to theLi⁺ ions, which limits the power capability of the electrodes. Althoughsubstitution of a ion larger S²⁻ ion for the O²⁻ ion increases the freevolume available to the Li⁺ ions, it also reduces the output voltage ofan elementary cell.

A host material that will provide a larger free volume for Li⁺-ionmotion in the interstitial space would allow realization of a higherlithium-ion conductivity σ_(Li), and hence higher power densities. Anoxide is needed for output voltage, and hence higher energy density. Aninexpensive, non-polluting transition-metal atom within the hoststructure would make the battery environmentally benign.

SUMMARY OF THE INVENTION

The present invention meets these goals more adequately than previouslyknown secondary battery cathode materials by providing oxides containinglarger tetrahedral oxide polyanions forming 3D framework host structureswith octahedral-site transition-metal oxidant cations, such as iron,that are environmentally benign.

The present invention provides electrode material for a rechargeableelectrochemical cell comprising an anode, a cathode and an electrolyte.The cell may additionally include an electrode separator. As usedherein, “electrochemical cell” refers not only to the building block, orinternal portion, of a battery but is also meant to refer to a batteryin general. Although either the cathode or the anode may comprise thematerial of the invention, the material will preferably be useful in thecathode.

Generally, in one aspect, the invention provides an ordered olivinecompound having the general formula LiMPO₄, where M is at least onefirst-row transition-metal cation. The alkali ion Li⁺ may beinserted/extracted reversibly from/to the electrolyte of the batteryto/from the interstitial space of the host MPO₄ framework of theordered-olivine structure as the transition-metal M cation (orcombination of cations) is reduced/oxidized by charge-compensatingelectrons supplied/removed by the external circuit of the battery in,for a cathode material, a discharge/charge cycle. In particular, M willpreferably be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples ofcombinations of the transition-metals for use as the substituent Minclude, but are not limited to, Fe_(1−x)Mn_(x), and Fe_(1−x)Ti_(x),where 0<x<1.

Preferred formulas for the ordered olivine electrode compounds of theinvention include, but are not limited to LiFePO₄, LiMnPO₄, LiCoPO₄,LiNiPO₄, and mixed transition-metal compounds such asLi_(1−2x)Fe_(1−x)Ti_(x)PO₄ or LiFe_(1−x)Mn_(x)PO₄, where 0<x<1. However,it will be understood by one of skill in the art that other compoundshaving the general formula LiMPO₄ and an ordered olivine structure areincluded within the scope of the invention.

The electrode materials of the general formula LiMPO₄ described hereintypically have an ordered olivine structure having a plurality of planesdefined by zigzag chains and linear chains, where the M atoms occupy thezigzag chains of octahedra and the Li atoms occupy the linear chains ofalternate planes of octahedral sites.

The present invention additionally provides electrode material for arechargeable electrochemical cell including an anode, a cathode and anelectrolyte where the material has a modified olivine structure. Thepristine olivine structure of LiMPO₄ may be modified either on theanionic site or on the cationic site to provide an alternative lithiuminsertion-type. It is also envisioned that the pristine olivinestructure may be modified on both the anionic and the cationic sites.Preferably, the structure is modified by aliovalent or isochargesubstitutions to provide better lithium ion diffusivity and electronicconductivity.

In general, “isocharge substitutions” refers to substitution of oneelement on a given crystallographic site with an element having asimilar charge. For example, Mg²⁺ is considered similarly isocharge withFe²⁺ and V⁵⁺ is similarly isocharge with P⁵⁺. Likewise, PO₄ ³⁻tetrahedra can be substituted with VO₄ ³⁻ tetrahedra. “Aliovalentsubstitution” refers to substitution of one element on a givencrystallographic site with an element of a different valence or charge.One example of an aliovalent substitution would be Cr³⁺ or Ti⁴⁺ on anFe²⁺ site. Another example would be Li⁺ on a Fe²⁺ site. These cathodematerials will generally have an olivine structure based on iron ormanganese derivatives whose general formula is:

Li_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[Si0₄]_(s)[VO₄]_(v)

where:

-   -   M may be Fe²⁺ or Mn²⁺ or mixtures thereof;    -   D may be a metal in the +2 oxidation state, preferably Mg²⁺,        Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺, or Ti²⁺;    -   T may be a metal in the +3 oxidation state, preferably Al³⁺,        Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, Zn³⁺, or V³⁺;    -   Q may be a metal in the +4 oxidation state, preferably Ti⁴⁺,        Ge⁴⁺, Sn⁴⁺, or V⁴⁺; and    -   R may be a metal in the +5 oxidation state, preferably V⁵⁺,        Nb⁵⁺, or Ta⁵⁺.

In this preferred embodiment, M, D, T, Q and R reside in octahedralsites. The additional coefficients may be defined as follows: xrepresents the degree of intercalation during operation of the electrodematerial; y represents the fraction of lithium ions on the (for example)initial Fe²⁺ sites; d represents the fraction of divalent ions (noted asD) on the initial Fe²⁺ sites; t represents the fraction of trivalentions (noted as T) on the initial Fe²⁺ sites; q represents the fractionof tetravalent ions (noted as Q) on the initial Fe²⁺ sites; r representsthe fraction of pentavalent ions (noted as R) on the initial Fe²⁺ sites;p represents the fraction of hexavalent sulfur (as discrete SO₄ ²⁻tetrahedra) on the initial P⁵⁺ sites; s represents the fraction oftetravalent silicon (as discrete SiO₄ ²⁻ tetrahedra) on the initial P⁵⁺sites; and v represents the fraction of pentavalent vanadium ions on theinitial P⁵⁺ sites.

The conditions for site occupancy and electroneutrality imply thefollowing:

0≦x≦1;

y+d+t+q+r≦1;

p+s+v≦1; and

3+s−p=x−y+2+t+2q+3r.

x, y, d, t, q, r, p, s, and v may be between 0 (zero) and 1 (one), withat least one of y, d, t, q, r, p, s, or v differing from 0. In apreferred embodiment y, d, t, q, r, and v may vary between 0 (zero) and0.2 (2/10) and p and s may vary between 0 (zero) and 0.5 (1/2).

The invention further provides an electrochemical cell or battery havingat least one positive and one negative electrode. At least one positiveelectrode in this embodiment contains theLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[Si0₄]_(s)[VO₄]_(v)material described above. Further, at least one negative electrodecontains a source of lithium ion at a high chemical activity. The phrase“high chemical activity” is generally understood in the art to refer toan electrode whose mean voltage during operation is not more positivethan 2 volts versus the Li⁺/Li⁰ couple.

Preferably, at least one negative electrode will contain metalliclithium, a lithium alloy, a lithium-carbon intercalation compound, alithium-transition metal mixed nitride of antifluorite, or alithium-titanium spinel Li_(1+x+z)Ti_(2−x)O₄, where 0≦x≦1/3 and0≦z≦1−2x. It will be understood by those of skill in the art that theterm “a” used before a compound encompasses structures containing morethan one of that type of compound. For example, “a lithium-transitionmetal mixed nitride of antifluorite” encompasses mixtures of more thanone of this type of compound and “a lithium-titanium spinel” encompassessolid solutions and/or mixtures of more than one of this type ofcompound with other spinels.

Alternatively, the electrochemical cell of the invention may contain anintercalation material with fast diffusion kinetics in the positiveelectrode containing theLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]_(v)material described above. The phrase “fast diffusion kinetics” isgenerally understood in the art as referring to a material able tosustain a specific current of at least 10 mA per gram of material withmore than 80% utilization of the capacity at the temperature ofoperation. Preferably, the intercalation material with fast diffusionkinetics may be a lamellar dichalcognenide, a vanadium oxide VO_(x)where 2.1≦x≦2.5, or a NASICON-related material such as Li₃Fe₂(PO₄)₃ orLi₃—Fe_(2−x)Ti_(x)(PO₄)₃ where x represents the degree of substitutionof Fe³⁺ by Ti⁴⁺.

In other preferred aspects, the electrochemical cell of the inventionwill include a conductive additive in at least one positive electrode.The conductive additive may preferably be carbon.

In other aspects, it is envisioned that the electrochemical cell of theinvention includes at least one positive electrode containing theLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]_(v)material described above and a polymeric binder. In certain preferredaspects, this positive electrode may additionally include a conductiveadditive, such as carbon.

Preferably, the polymeric binder may be a homopolymer or copolymer oftetrafluoroethylene, an ethylene-propylene-diene terpolymer, apolyether, a polyester, a methylmethacrylate-based polymer, anacrylonitrile-based polymer, or a vinylidene fluoride-based polymer. Itis contemplated that the polymeric binder for use in conjunction withthe present invention may be crosslinked, but those of skill in the artwill appreciate that cross-linkage is not essential. The term“crosslinked” refers to the presence of physical or chemical bondsbetween the polymer chains. Generally, those skilled in the art measurecrosslinkage in terms of the number of crosslinks per cubic centimeter.The polymeric binder for use in conjunction with the present inventionwill preferably have a cross-linkage of between 10¹⁸ and 10²⁰inter-chain bonds per cubic centimeter.

Alternatively, the polymeric binder may possess ionic conductivity.Ionic conductivity is generally understood in the art to be the abilityto carry a current due to the motion of ions. Preferred values of ionicconductivity are between about 10⁻⁷ and about 10⁻² (Scm⁻¹). In certainembodiments, the polymeric binder may be swollen by an aprotic solventwhich contains a salt, the cation of which is at least in part Li⁺. Theaprotic solvent may preferably be ethylene carbonate, propylenecarbonate, dimethylcarbonate, diethylcarbonate, methyl-ethylcarbonate,γ-butyrolactone, a tetraalkylsulfamide, or a dialkylether of a mono-,di-, tri-, tetra- or higher oligo-ethylene glycol of molecular weightlower or equal to 2000 and mixtures thereof.

The invention further provides a variable optical transmission devicewhich is constructed from transparent semi-conductor coated glass orplastic and includes two electrodes separated by a solid or gelelectrolyte. In this embodiment, at least one electrode contains theLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]_(v)material as described above. Preferably, theLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]_(v)compound of the invention is layered on a transparent semiconductorcoated glass or plastic in a thin film. It is preferred that thesemi-conductor be coated onto the glass or plastic at a thickness ofbetween 200 and 10⁴ Angstroms (Å) or between 20 and 10³ nanometers (nm).The material of the invention may be placed onto the glass or plastic,for example, using a vacuum deposition technique, by sputtering, or froma sol-gel precursor. The techniques for placing the compound of theinvention onto the glass or plastic are well known to those skilled inthe art. Preferred techniques include sputtering, chemical vapordeposition (CVD) from organometallic precursors like metalhexafluoroacetylacetonates and organic phosphates or silicates, sol-gelfrom hydrolysis-condensation of metal alkoxides in water-organicsolutions in the presence of phosphoric acid and organosiloxanes.

It is preferred that the glass for use in conjunction with the presentinvention be conventional optical quality glazing. Preferred plasticsinclude high transparency, high mechanical strength material likepolyethylene terephthalate (Mylar®). The transparent semi-conductor iscontemplated to be virtually any transparent semi-conductor but ispreferably antimony- or fluorine-doped tin oxide, tin- or fluorine-dopedindium oxide, or non-stoichiometric zinc oxide.

In another aspect, the invention provides electrode materials for arechargeable electrochemical cell comprising an anode, a cathode and anelectrolyte, with or without an electrode separator, where the electrodematerials comprise a rhombohedral NASICON material having the formulaY_(x)M₂(PO₄)₃, where 0≦x≦5. Preferably, the compounds of the inventionwill be useful as the cathode of a rechargeable electrochemical cell.The alkali ion Y may be inserted from the electrolyte of the battery tothe interstitial space of the rhombohedral M₂(XO₄)₃ NASICON hostframework as the transition-metal M cation (or combination of cations)is reduced by charge-compensating electrons supplied by the externalcircuit of the battery during discharge with the reverse processoccurring during charge of the battery. While it is contemplated thatthe materials of the invention may consist of either a singlerhombohedral phase or two phases, e.g. orthorhombic and monoclinic, thematerials are preferably single-phase rhombohedral NASICON compounds.Generally, M will be at least one first-row transition-metal cation andY will be Li or Na. In preferred compounds, M will be Fe, V, Mn, or Tiand Y will be Li.

Redox energies of the host M cations can be varied by a suitable choiceof the XO₄ polyanion, where X is taken from Si, P, As, or S and thestructure may contain a combination of such polyanions. Tuning of theredox energies allows optimization of the battery voltage with respectto the electrolyte used in the battery. The invention replaces the oxideion O²⁻ of conventional cathode materials by a polyanion (XO₄)^(m−) totake advantage of (1) the larger size of the polyanion, which canenlarge the free volume of the host interstitial space available to thealkali ions, and (2) the covalent X—O bonding, which stabilizes theredox energies of the M cations with M-O—X bonding so as to createacceptable open-circuit voltages V_(oc) with environmentally benignFe³⁺/Fe²⁺ and/or Ti⁴⁺/Ti³⁺ or V⁴⁺/V³⁺ redox couples.

Preferred formulas for the rhombohedral NASICON electrode compounds ofthe invention include, but are not limited to those having the formulaLi_(3+x)Fe₂(PO₄)₃, Li_(2+x)FeTi(PO₄)₃, Li_(x)TiNb(PO₄)₃, andLi_(1+x)FeNb(PO₄)₃, where 0<x<2. It will be understood by one of skillin the art that Na may be substituted for Li in any of the abovecompounds to provide cathode materials for a Na ion rechargeablebattery. For example, one may employ Na_(3+x)Fe₂(PO₄)₃,Na_(2+x)FeTi(PO₄)₃, Na_(x)TiNb(PO₄)₃ or Na_(1+x)FeNb(PO₄)₃, where 0<x<2,in a Na ion rechargeable battery. In this aspect, Na⁺ is the working ionand the anode and electrolyte comprise a Na compound.

Compounds of the invention having the rhombohedral NASICON structureform a framework of MO₆ octahedra sharing all of their corners with XO₄tetrahedra (X=Si, P, As, or S), the XO₄ tetrahedra sharing all of theircorners with octahedra. Pairs of MO₆ octahedra have faces bridged bythree XO₄ tetrahedra to form “lantern” units aligned parallel to thehexagonal c-axis (the rhombohedral [111] direction), each of these XO₄tetrahedra bridging to two different “lantern” units. The Li⁺ or Na⁺ions occupy the interstitial space within the M₂(XO₄)₃ framework.Generally, Y_(x)M₂(XO₄)₃ compounds with the rhombohedral NASICONframework may be prepared by solid-state reaction of stoichiometricproportions of the Y, M, and XO₄ groups for the desired valence of the Mcation. Where Y is Li, the compounds may be prepared indirectly from theNa analog by ion exchange of Li⁺ for Na⁺ ions in a molten LiNO₃ bath at300° C. For example, rhombohedral LiTi₂(PO₄)₃ may be prepared fromintimate mixtures of Li₂CO₃ or LiOH.H₂O, TiO₂, and NH₄H₂PO₄.H₂O calcinedin air at 200° C. to eliminate H₂O and CO₂ followed by heating in airfor 24 hours near 850° C. and a further heating for 24 hours near 950°C. However, preparation of Li₃Fe₂(PO₄)₃ by a similar solid-statereaction gives the undesired monoclinic framework. To obtain therhombohedral form, it is necessary to prepare rhombohedral Na₃Fe₂(PO₄)₃by solid-state reaction of NaCO₃, Fe{CH₂COOH}₂ and NH₄H₂PO₄.H₂O, forexample. The rhombohedral form of Li₃Fe₂(PO₄)₃ is then obtained at 300°C. by ion exchange of Li⁺ for Na⁺ in a bath of molten LiNO₃. It will beunderstood by one of skill in the art that the rhombohedral Na compoundswill be useful as cathode materials in rechargeable Na ion batteries.

In another aspect of the invention, the rhombohedral NASICON electrodecompounds may have the general formula Y_(x)M₂(PO₄)_(y)(XO₄)_(3−y),where 0<y≦3, M is a transition-metal atom, Y is Li or Na, and X=Si, As,or S and acts as a counter cation in the rhombohedral NASICON frameworkstructure. In this aspect, the compound comprises a phosphate anion asat least part of an electrode material. In preferred embodiments, thecompounds are used in the cathode of a rechargeable battery. Preferredcompounds having this general formula include, but are not limited toLi_(1+x)Fe₂(SO₄)₂(PO₄), where 0≦x≦1.

The rhombohedral NASICON compounds described above may typically beprepared by preparing an aqueous solution comprising a lithium compound,an iron compound, a phosphate compound and a sulfate compound,evaporating the solution to obtain dry material and heating the drymaterial to about 500° C. Preferably, the aqueous starting solutioncomprises FeCl₃, (NH₄)₂SO₄, and LiH₂PO₄.

In a further embodiment, the invention provides electrode materials fora rechargeable electrochemical cell comprising an anode, a cathode andan electrolyte, with or without an electrode separator, where theelectrode materials have a rhombohedral NASICON structure with thegeneral formula A_(3−x)V₂(PO₄)₃. In these compounds, A may be Li, Na ora combination thereof and 0≦x≦2. In preferred embodiments, the compoundsare a single-phase rhombohedral NASICON material. Preferred formulas forthe rhombohedral NASICON electrode compounds having the general formulaA_(3−x)V₂(PO₄)₃ include, but are not limited to those having the formulaLi_(2−x)NaV₂(PO₄)₃, where 0≦x≦2.

The rhombohedral NASICON materials of the general formulaA_(3−x)V₂(PO₄)₃ may generally be prepared by the process outlined inFIG. 9. Alternatively, Li₂NaV₂(PO₄)₃ may be prepared by a directsolid-state reaction from LiCO₃, NaCO₃, NH₄H₂PO₄.H₂O and V₂O₃.

In a further aspect, the invention provides a secondary (rechargeable)battery where an electrochemical cell comprises two electrodes and anelectrolyte, with or without an electrode separator. The electrodes aregenerally referred to as the anode and the cathode. The secondarybatteries of the invention generally comprise as electrode material, andpreferably as cathode material, the compounds described above. Moreparticularly, the batteries of the invention have a cathode comprisingthe ordered olivine compounds described above or the rhombohedralNASICON compounds described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to demonstrate further certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1. FIG. 1 shows a typical polarization curve for the batteryvoltage V vs. the I delivered across a load. The voltage drop(V_(oc)−V)≡η(I) of a typical curve is a measure of the batteryresistance Rb(I). The interfacial voltage drops saturate in region (i).The slope of the curve in region (ii) is dV/dI≈R_(e1)+R_(c)(A)+R_(c)(C),the sums of the electrolyte resistance R_(e1) and the current-collectorresistances at the anode and cathode. Region (iii) is diffusion-limited.At the higher currents I, normal processes do not bring ions to orremove them from the electrode/electrolyte interfaces rapidly enough tosustain an equilibrium reaction.

FIGS. 2A, 2B and 2C. FIG. 2A shows discharge/charge curves at 0.05mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄ as cathode andlithium as anode. A plateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redoxcouple relative to the lithium anode. FIG. 2B shows discharge/chargecurves at 0.05 mA·cm⁻² (1.13 mA·g⁻¹) for the olivineLi_(1−x)Fe_(0.5)Mn_(0.5)PO₄ as cathode relative to a lithium anode. Aplateau at 3.4V corresponds to the Fe³⁺/Fe²⁺ redox couple relative tothe lithium anode. A plateau at 4.1 V corresponds to the Mn³⁺/Mn²⁺couple. FIG. 2C shows discharge/charge curves vs. lithium at 0.05mA·cm⁻² (0.95 mA·g⁻¹) for the olivine Li_(1−x)FePO₄.

FIG. 3. FIG. 3 shows discharge/charge curves of anFePO₄/LiClO₄+PC+DME/Li coin cell at 185 mA·g for FePO₄ prepared bychemical extraction of Li (delithiation) from LiFePO₄.

FIG. 4. FIG. 4 shows a schematic representation of the motion ofLiFePO₄/FePO₄ interface on lithium insertion in to a particle of FePO₄.

FIGS. 5A and 5B. FIG. 5A shows the rhombohedral R3c (NASICON) frameworkstructure of Li₃Fe₂(PO₄)₃ prepared by ion exchange from Na₃Fe₂(PO₄)₃;FIG. 5B shows the monoclinic P2₁/n framework structure of Li₃Fe₂(PO₄)₃prepared by solid-state reaction. The large, open three-dimensionalframework of FeO₆ octahedra and PO₄ tetrahedra allows an easy diffusionof the lithium ions.

FIGS. 6A and 6B. FIG. 6A shows discharge/charge curves vs. lithium at0.1 mA·cm⁻² for rhombohedral Li_(3+x)Fe₂(PO₄)₃ where 0<x<2. The shape ofthe curve for lithium insertion into rhombohedral Li_(3+x)Fe₂(PO₄)₃ issurprisingly different from that for the monoclinic form. However, theaverage V_(oc) at 2.8 V remains the same. The Li⁺-ion distribution inthe interstitial space appears to vary continuously with x with a highdegree of disorder. FIG. 6B shows discharge/charge curves vs. lithium at0.1 mA·cm⁻² for monoclinic Li_(3+x)Fe₂(PO₄)₃ where 0<x<2.

FIGS. 7A and 7B. FIG. 7A shows discharge curves vs. a lithium anode atcurrent densities of 0.05-0.5 mA·cm⁻² for rhombohedralLi_(3+x)Fe₂(PO₄)₃. A reversible capacity loss on increasing the currentdensity from 0.05 to 0.5 mA·cm⁻² is shown. This loss is much reducedcompared to what is encountered with the monoclinic system. FIG. 7Bshows discharge curves at current densities of 0.05-0.5 mA·cm⁻² formonoclinic Li_(3+x)Fe₂(PO₄)₃.

FIG. 8. FIG. 8 shows discharge/charge curves at 0.05 mA·cm⁻² (0.95mA·g⁻¹) for the rhombohedral Li₂NaV₂(PO₄)₃. Rhombohedral Li₂NaV₂(PO₄)₃reversibly intercalates 1.5 Li per formula unit for a discharge capacityof 100 mAh·g⁻¹ with average closed-circuit voltage of 3.8 V vs. alithium anode.

FIG. 9. FIG. 9 illustrates the solid-state synthesis of Li₂NaV₂(PO₄)₃having the rhombohedral NASICON framework.

FIG. 10. FIG. 10 shows discharge/charge curves vs. lithium at 0.1mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄)(SO₄)₂ where 0≦x≦2.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Present-day secondary (rechargeable) lithium batteries use a solidreductant as the anode, or negative electrode, and a solid oxidant asthe cathode, or positive electrode. It is important that the chemicalreactions at the anode and cathode of a lithium secondary battery bereversible. On discharge, the metallic anode supplies Li⁺ ions to theLi⁺-ion electrolyte and electrons to the external circuit. The cathodeis a host compound into/from which the working Li⁺ ion of theelectrolyte can be inserted/extracted reversibly as a guest species overa large solid-solubility range (Goodenough 1994). When the Li⁺ ions areinserted as a guest species into the cathode, they arecharge-compensated by electrons from the external circuit. On charge,the removal of electrons from the cathode by an external field releasesLi⁺ ions back to the electrolyte to restore the parent host structure.The resultant addition of electrons to the anode by the external fieldattracts charge-compensating Li⁺ ions back into the anode to restore itto its original composition.

The present invention provides new materials for use as cathodes inlithium secondary (rechargeable) batteries. It will be understood thatthe anode for use with the cathode material of the invention may be anylithium anode material, such as a reductant host for lithium orelemental lithium itself. Preferably, the anode material will be areductant host for lithium. Where both the anode and cathode are hostsfor the reversible insertion or removal of the working ion into/from theelectrolyte, the electrochemical cell is commonly called a“rocking-chair” cell. An implicit additional requirement of a secondarybattery is maintenance not only of the electrode/electrolyte interfaces,but also of electrical contact between host particles, throughoutrepeated discharge/recharge cycles.

Since the volumes of the electrode particles change as a result of thetransfer of atoms from one to another electrode in a reaction, thisrequirement normally excludes the use of a crystalline or glassyelectrolyte with a solid electrode. A non-aqueous liquid or polymerelectrolyte having a large energy-gap window between its highestoccupied molecular orbital (HOMO) and its lowest unoccupied molecularorbital (LUMO) is used with secondary lithium batteries in order torealize higher voltages. For example, practical quantities of very ioniclithium salts such as LiClO₄, LiBF₄ and LiPF₆ can be dissolved inempirically optimized mixtures of propylene carbonate (PC), ethylenecarbonate (EC), or dimethyl carbonate (DMC) to provide acceptableelectrolytes for use with the cathodes of the invention. It will berecognized by those of skill in the art that the (ClO₄)⁻ anion isexplosive and not typically suitable for commercial applications.

General Design Considerations

The power output P of a battery is the product of the electric current Idelivered by the battery and the voltage V across the negative andpositive posts (equation 1).

P=IV  (1)

The voltage V is reduced from its open-circuit value V_(oc) (I=0) by thevoltage drop IR_(b) due to the internal resistance R_(b) of the battery(equation 2).

V=V _(oc) −IR _(b)  (2)

The open-circuit value of the voltage is governed by equation 3.

V _(oc)=(μ_(A)−μ_(C))/(−nF)<5V  (3)

In equation 3, n is the number of electronic charges carried by theworking ion and F is Faraday's constant. The magnitude of theopen-circuit voltage is constrained to V_(oc)<5V not only by theattainable difference μ_(A)−μ_(c) of the electrochemical potentials ofthe anode reductant and the cathode oxidant, but also by the energy gapE_(g) between the HOMO (highest occupied molecular orbital) and the LUMO(lowest unoccupied molecular orbital) of a liquid electrolyte or by theenergy gap E_(g) between the top of the valence band and the bottom ofthe conduction band of a solid electrolyte.

The chemical potential μ_(A), which is the Fermi energy ε_(F) of ametallic-reductant anode or the HOMO of a gaseous or liquid reductant,must lie below the LUMO of a liquid electrolyte or the conduction bandof a solid electrolyte to achieve thermodynamic stability againstreduction of the electrolyte by the reductant. Similarly, the chemicalpotential μ_(A)−μ_(C), which is the LUMO of a gaseous or liquid oxidantor the Fermi energy of a metallic-oxidant cathode, must lie above theHOMO of a liquid electrolyte or the valence band of a solid electrolyteto achieve thermodynamic stability against oxidation of the electrolyteby the oxidant. Thermodynamic stability thus introduces the constraint

μ_(A)−μ_(C) ≦E _(g)  (4)

as well as the need to match the “window” E_(g) of the electrolyte tothe energies μ_(A) and μ_(C) of the reactants to maximize V_(oc). Itfollows from equations 1 and 2 that realization of a high maximum powerP_(max) (equation 5) requires, in addition to as high a V_(oc) aspossible, a low internal battery resistance Rb (see equation 6).

P _(max) =I _(max) V _(max)  (5)

R _(b) =R _(e1) +R _(in)(A)+R _(in)(C)+R _(c)(A)+R _(c)(C)  (6)

The electrolyte resistance R_(e1) to the ionic current is proportionalto the ratio of the effective thickness L to the geometrical area A ofthe interelectrode space that is filled with an electrolyte of ionicconductivity σ_(i) (equation 7).

R _(e1)=(L/σ _(i) A)  (7)

Since ions move diffusively, σ_(i) (see equation 8) increases withtemperature. A σ_(i)≦0.1 Scm⁻¹ (the maximum σ_(i) represents theroom-temperature protonic conductivity σ_(H) in a strong acid) at anoperating temperature T_(op) dictates the use of a membrane separator oflarge geometrical area A and small thickness L.

σ_(Li)=(B/T)exp(−E _(v) /kT)  (8)

The resistance to transport of the working ion across theelectrolyte-electrode interfaces is proportional to the ratio of thegeometrical and interfacial areas at each electrode:

R _(in) ˜A/A _(in)  (9)

where the chemical reaction of the cell involves ionic transport acrossan interface, equation 9 dictates construction of a porous,small-particle electrode. Achievement and retention of a high electrodecapacity, i.e., utilization of a high fraction of the electrode materialin the reversible reaction, requires the achievement and retention ofgood electronic contact between particles as well as a largeparticle-electrolyte interface area over many discharge/charge cycles.If the reversible reaction involves a first-order phase change, theparticles may fracture or lose contact with one another on cycling tobreak a continuous electronic pathway to the current collector.

Loss of interparticle electrical contact results in an irreversible lossof capacity. There may also be a reversible capacity fade. Where thereis a two-phase process (or even a sharp guest-species gradient at adiffusion front) without fracture of the particles, the area of theinterface (or diffusion front) decreases as the second phase penetratesthe electrode particle. At a critical interface area, diffusion acrossthe interface may not be fast enough to sustain the current I, so notall of the particle is accessible. The volume of inaccessible electrodeincreases with I, which leads to a diffusion-limited reversible capacityfade that increases with I. This problem becomes more important at lowerionic conductivity σ_(Li).

The battery voltage V vs. the current I delivered across a load iscalled the polarization curve. The voltage drop (V_(oc)−V)≡η(I) of atypical curve, FIG. 1, is a measure of the battery resistance (seeequation 10).

R _(b)(I)=η(I)/I  (10)

On charging, η(I)=(V_(app)−V_(oc)) is referred to as an overvoltage. Theinterfacial voltage drops saturate in region (i) of FIG. 1; therefore inregion (ii) the slope of the curve is

dV/dI=R _(e1) +R _(c)(A)+R _(c)(C)  (11)

Region (iii) is diffusion-limited; at the higher currents I, normalprocesses do not bring ions to or remove them from theelectrode/electrolyte interfaces rapidly enough to sustain anequilibrium reaction.

The battery voltage V vs. the state of charge, or the time during whicha constant current I has been delivered, is called a discharge curve.

Cathode Materials

The cathode, or positive electrode, material of the present invention,for use in a secondary lithium battery, consists of a host structureinto which lithium can be inserted reversibly. The maximum power output,P_(max) (see equation 5) that can be achieved by a cell depends on theopen-circuit voltage V_(oc)=ΔE/e and the overvoltage η(I) at the currentI_(max) of maximum power

V _(max) =V _(oc)−η(I)  (12)

ΔE is the energy difference between the work function of the anode (orthe HOMO of the reductant) and that of the cathode (or the LUMO of theoxidant). In order to obtain a high V_(oc), it is necessary to use acathode that is an oxide or a halide. It is preferable that the cathodebe an oxide in order to achieve a large V_(oc) and good electronicconductivity. To minimize η(I_(max)), the electrodes must be goodelectronic as well as ionic conductors and they must offer a lowresistance to mass transfer across the electrode/electrolyte interface.To obtain a high I_(max), it is necessary to have a largeelectrode/electrolyte surface area. In addition, where there is atwo-phase interface within the electrode particle, the rate of masstransfer across this interface must remain large enough to sustain thecurrent. This constraint tends to limit the electrode capacity more asthe current increases.

Oxide host structures with close-packed oxygen arrays may be layered, asin Li_(1−x)CoO₂(Mizushima, et al. 1980), or strongly bonded in threedimensions (3D) as in the manganese spinels Li_(1−x)[Mn₂]O₄ (Thackeray1995; Thackeray et al. 1983; Thackeray et al. 1984; Guyomard andTarascon 1992; and Masquelier et al. 1996). Li intercalation into a vander Waals gap between strongly bonded layers may be fast, but it canalso be accompanied by unwanted species from a liquid electrolyte. Onthe other hand, strong 3D bonding within a close-packed oxygen array, asoccurs in the spinel framework [Mn₂]O₄, offers too small a free volumefor the guest Li⁺ ions to have a high mobility at room temperature,which limits I_(max). Although this constraint in volume of theinterstitial space makes the spinel structure selective for insertion ofLi⁺ ions, it reduces the Li⁺-ion mobility and hence Li⁺-ion conductivityσ_(Li). The oxospinels have a sufficiently high σ_(Li), to be usedcommercially in low-power cells (Thackeray et al., 1983) but would notbe acceptable for the high power cells of the insertion.

The present invention overcomes these drawbacks by providing cathodematerials containing larger tetrahedral polyanions which form 3Dframework host structures with octahedral-site transition-metal oxidantcations. In the cathode materials of the invention having the NASICONstructure, the transition-metal ions are separated by the polyanions, sothe electronic conductivity is polaronic rather than metallic.Nevertheless, the gain in σ_(Li) more than offsets the loss inelectronic conductivity.

Variation of the energy of a given cation redox couple from one compoundto another depends on two factors: (a) the magnitude of the crystallineelectric field at the cation, which may be calculated for a purely ionicmodel by a Madelung summation of the Coulomb fields from the other ionspresent, and (b) the covalent contribution to the bonding, which may bemodulated by the strength of the covalent bonding at a nearest-neighborcounter cation. The stronger is the negative Madelung potential at acation, the higher is a given redox energy of a cation. Similarly thestronger is the covalent bonding of the electrons at a transition-metalcation, the higher is a given redox energy of that cation. The lower theredox energy of the cation host transition-metal ion, the larger isV_(oc).

The redox couples of interest for a cathode are associated withantibonding states of d-orbital parentage at transition-metal cations Mor 4f-orbital parentage at rare-earth cations Ln in an oxide. Thestronger is the cation-anion covalent mixing, the higher is the energyof a given LUMO/HOMO redox couple. Modulation of the strength of thecation-anion covalence at a given M or Li cation by nearest-neighborcations that compete for the same anion valence electrons is known asthe inductive effect. Changes of structure alter primarily the Madelungenergy as is illustrated by raising of the redox energy within a spinel[M₂]O₄ framework by about 1 eV on transfer of Li⁺ ions from tetrahedralto octahedral interstitial sites. Changing the counter cation, but notthe structure, alters primarily the inductive effect, as is illustratedby a lowering of the Fe³⁺/Fe²⁺ redox energy by 0.6 eV on changing(MoO₄)²⁻ or (WO₄)²⁻ to (SO₄)²⁻ polyanions in isostructural Fe₂(XO₄)₃compounds. Raising the energy of a given redox couple in a cathodelowers the voltage obtained from cells utilizing a common anode.Conversely, raising the redox energy of an anode raises the cell voltagewith respect to a common cathode.

The invention provides new cathode materials containing oxidepolyanions, including the oxide polyanion (PO₄)³⁻ as at least oneconstituent, for use in secondary (rechargeable) batteries. For example,the cathode materials of the present invention may have the generalformula LiM(PO₄) with the ordered olivine structure, the modifiedolivine structure of LiM(PO₄), or the more open rhombohedral NASICONframework structure. The cathode materials of the present invention havethe general formula LiM(PO₄) for the ordered olivine structure, thegeneral formulaLi_(x+y)M_(1−(y+d+t+q+r))D_(d)T_(t)Q_(q)R_(r)[PO₄]_(1−(p+s+v))[SO₄]_(p)[SiO₄]_(s)[VO₄]_(v)for the modified olivine structure or Y_(x)M₂(PO₄)_(y)(XO₄)_(3−y) forthe rhombohedral NASICON framework structure. The parameters for themodified olivine structure are as follows:

-   -   M may be Fe²⁺ or Mn²⁺ or mixtures thereof;    -   D may be a metal in the +2 oxidation state, preferably Mg²⁺,        Ni²⁺, Co²⁺, Zn²⁺, Cu²⁺, or Ti²⁺;    -   T may be a metal in the +3 oxidation state, preferably Al³⁺,        Ti³⁺, Cr³⁺, Fe³⁺, Mn³⁺, Ga³⁺, Zn³⁺, or V³⁺;    -   Q may be a metal in the +4 oxidation state, preferably Ti⁴⁺,        Ge⁴⁺, Sn⁴⁺, or V⁴⁺;    -   R may be a metal in the +5 oxidation state, preferably V⁵⁺,        Nb⁵⁺, or Ta⁵⁺.

In this preferred embodiment x is the degree of intercalation duringoperation of the electrode material; y is the fraction of lithium ionson the initial Fe²⁺ sites; d is the fraction of divalent ions (noted asD) on the initial Fe²⁺ sites; t is the fraction of trivalent ions (notedas T) on the initial Fe²⁺ sites; q is the fraction of tetravalent ions(noted as Q) on the initial Fe²⁺ sites; r is the fraction of pentavalentions (noted as R) on the initial Fe²⁺ sites; p is the fraction ofhexavalent sulfur (as discrete SO₄ ²⁻ tetrahedra) on the initial P⁵⁺sites; and s is the fraction of tetravalent silicon (as discrete SiO₄ ²⁻tetrahedra) on the initial sites; v is the stoichiometric coefficientfor V⁵⁺ residing in tetrahedral sites; and M, D, T, Q and R reside inoctahedral sites.

The conditions for site occupancy and electroneutrality imply:

0≦x≦1;

y+d+t+q+r≦1;

p+s+v≦1; and

3+s−p=x−y+2+t+2q+3r.

Generally, x, y, d, t, q, r, p, s, and v may be between 0 (zero) and 1(one), with at least one of y, d, t, q, r, p, s, or v differing from 0.Preferably, y, d, t, q, r and v may vary between 0 (zero) and 0.2 (2/10)and p and s may vary between 0 (zero) and 0.5 (1/2).

In the rhombohedral NASICON framework structure, 0<y≦3, M is atransition-metal atom, Y is Li or Na and X=Si, As or S and acts as acounter cation.

The olivine structure of Mg₂SiO₄ consists of a slightly distorted arrayof oxygen atoms with Mg²⁺ ions occupying half the octahedral sites intwo different ways. In alternate basal planes, they form zigzag chainsof corner-shared octahedra running along the c-axis and in the otherbasal planes they form linear chains of edge-shared octahedra runningalso along the c-axis.

In the ordered LiMPO₄ olivine structures of the invention, the M atomsoccupy the zigzag chains of octahedra and the Li atoms occupy the linearchains of the alternate planes of octahedral sites. In this embodimentof the present invention, M is preferably Mn, Fe, Co, Ni or combinationsthereof. Removal of all of the lithium atoms leaves the layeredFePO₄-type structure, which has the same Pbnm orthorhombic space group.These phases may be prepared from either end, e.g., LiFePO₄ (triphylite)or FePO₄ (heterosite), by reversible extraction or insertion of lithium.

FIG. 2A, FIG. 2B and FIG. 2C show discharge/charge curves vs. lithium at0.05 mA·cm⁻² (0.95 mA·g⁻¹ and 1.13 mA·g⁻¹, respectively) forLi_(1−x)FePO₄, Li_(1−x)Fe_(0.5)Mn_(0.5)PO₄ and LiFePO₄, respectively,where 0≦x≦0.5. A plateau at 3.4 V corresponds to the Fe³⁺/Fe²⁺ redoxcouple and a plateau at 4.1 V corresponds to the Mn³⁺/Mn²⁺ couple. WithLiClO₄ in PC and DME as the electrolyte, it is only possible to chargeup a cathode to 4.3 V vs. a lithium anode, so it was not possible toextract lithium from LiMnPO₄, LiCoPO₄ and LiNiPO₄ with this electrolyte.However, in the presence of iron, the Mn³⁺/Mn²⁺ couple becomesaccessible. The inaccessibility is due to the stability of theMn³⁺/Mn²⁺, Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ couples in the presence of thepolyanion (PO₄)³⁻. The relatively strong covalence of the PO₄tetrahedron of the compounds of the present invention stabilizes theredox couples at the octahedral sites to give the high V_(oc)'s that areobserved.

Insertion of lithium into FePO₄ was reversible over the several cyclesstudied. FIG. 3 shows discharge/charge curves of FePO₄/LiClO₄+PC+DME/Licoin cell at 185 mA·g⁻¹ for FePO₄ prepared by chemical extraction of Li(delithiation) from LiFePO₄. The Li_(x)FePO₄ material of the presentinvention represents a cathode of good capacity and containsinexpensive, environmentally benign elements. While a nearlyclose-packed-hexagonal oxide-ion array apparently provides a relativelysmall free volume for Li⁺-ion motion, which would seem to support onlyrelatively small current densities at room temperature, increasing thecurrent density does not lower the closed-circuit voltage V. Rather, itdecreases, reversibly, the cell capacity. Capacity is easily restored byreducing the current.

As illustrated schematically in FIG. 4, lithium insertion proceeds fromthe surface of the particle moving inwards behind a two-phase interface.In the system shown, it is a Li_(x)FePO₄/Li_(1−x)FePO₄ interface. As thelithiation proceeds, the surface area of the interface shrinks. For aconstant rate of lithium transport per unit area across the interface, acritical surface area is reached where the rate of total lithiumtransported across the interface is no longer able to sustain thecurrent. At this point, cell performance becomes diffusion-limited. Thehigher the current, the greater is the total critical interface areaand, hence, the smaller the concentration x of inserted lithium beforethe cell performance becomes diffusion-limited. On extraction oflithium, the parent phase at the core of the particle grows back towardsthe particle surface. Thus, the parent phase is retained on repeatedcycling and the loss in capacity is reversible on lowering the currentdensity delivered by the cell. Therefore, this loss of capacity does notappear to be due to a breaking of the electrical contact betweenparticles as a result of volume changes, a process that is normallyirreversible. Moreover, the problem of decrease of capacity due to theparticles' breaking of electrical contact may be overcome by reducingthe particle size of the cathode material to the nanometer scale.

Electrode materials with the olivine structure LiFePO₄ (triphylite) andthe quasi-isomorphous delithiated material FePO₄, have the advantage ofan operating voltage of 3.5V vs. Li⁺/Li⁰, i.e. in the stability windowof both liquid and polymer electrolytes with a flat discharge (lithiumintercalation) plateau (as seen in FIGS. 2A-2C). It may be possible toincrease the diffusion kinetics and electronic conductivity by using amodified olivine structure in an electrode. The absence ofnon-stoichiometry or mutual miscibility for both phases (LiFePO₄ andFePO₄) in the materials with a pristine olivine structure may contributeto lower electronic conductivity than the materials having a modifiedolivine structure and to slower diffusion kinetics than may be achievedusing materials with a modified olivine structure.

Thus, the invention additionally provides cathode materials where thepristine olivine structure of LiM(PO₄) (M=Fe or Mn or their solidsolutions) is modified either on the anionic site or on the cationicsite, or on both, by aliovalent or isocharge substitutions, to providebetter lithium ion diffusivity and electronic conductivity. Thesesubstitutions allow for the coexistence of iron or manganese in twodifferent oxidation states in the same phase. These substitutions mayfurther introduce specific interactions with other elements having redoxlevels close to those of Fe and Mn (e.g., Fe²⁺/Ti⁴⁺

Fe³⁺/Ti³⁺, Mn²⁺/V⁺

Mn³⁺/V⁴⁺, etc. . . . ). Both of these situations are favorable toelectronic conductivity. Further, disorder on the anionic site providespreferential diffusion sites for Li⁺. Similarly, partial substitution ofphosphorus by vanadium, or to some extent by silicon, increases thelattice parameters, thereby increasing the size of the bottlenecks whichtend to slow diffusion. The formation of non-stoichiometry domains withmixed valence states and/or transition-metal mediated electron hopping,as well as partial substitution of phosphorus sites, differentiates themodified olivine compounds from the LiMPO₄/MPO₄ compounds in which thetotality of Fe(Mn) is either in the +2 or +3 oxidation state.

The invention further provides new cathode materials exhibiting arhombohedral NASICON framework. NASICON, as used herein, is an acronymfor the framework host of a sodium superionic conductorNa_(1+3x)Zr₂(P₁−_(x)Si_(x)O₄)₃. The compound Fe₂(SO₄)₃ has two forms, arhombohedral NASICON structure and a related monoclinic form (Goodenoughet al. 1976; Long et al. 1979). Each structure contains units of twoFeO₆ octahedra bridged by three corner-sharing SO₄ tetrahedra. Theseunits form 3D frameworks by the bridging SO₄ tetrahedra of one unitsharing corners with FeO₆ octahedra of neighboring Fe₂(SO₄)₃ elementarybuilding blocks so that each tetrahedron shares corners with onlyoctahedra and each octahedron with only tetrahedra. In the rhombohedralform, the building blocks are aligned parallel, while they are alignednearly perpendicular to one another in the monoclinic phase. Thecollapsed monoclinic form has a smaller free volume for Li⁺-ion motionwhich is why the rhombohedral form is preferred. In these structures,the FeO₆ octahedra do not make direct contact, so electron transfer froman Fe²⁺ to an Fe³⁺ ion is polaronic and therefore activated.

Li_(x)Fe₂(SO₄)₃ has been reported to be a candidate material for thecathode of a Li⁺-ion rechargeable battery with a V_(oc)=3.6 V vs. alithium anode (Manthiram and Goodenough 1989). While the sulfates wouldseem to provide the desired larger free volume for Li, batteries usingsulfates in the cathode material tend to exhibit phase-transitionproblems, lowering the electronic conductivity. The reversible lithiuminsertion into both rhombohedral and monoclinic Fe₂(SO₄)₃ gives a flatclosed-circuit voltage vs. a lithium anode of 3.6 V (Manthiram andGoodenough 1989; Okada et al. 1994; Nanjundaswamy et al. 1996). Neitherparent phase has any significant solid solution with the orthorhombiclithiated phase Li₂Fe₂(SO₄)₃, which is derived from the rhombohedralform of Fe₂(SO₄)₃ by a displacive transition that leaves the frameworkintact. Powder X-ray diffraction verifies that lithiation occurs via atwo-phase process (Nanjundaswamy et al. 1996). Increasing the currentdensity does not change significantly the closed-circuit voltage V, butit does reduce reversibly the capacity. The reduction in capacity for agiven current density is greater for the motion of the lithiatedinterface. The interstitial space of the framework allows fast Li⁺-ionmotion, but the movement of lithium across the orthorhombic/monoclinicinterface is slower than that across the orthorhombic/rhombohedralinterface, which makes the reversible loss of capacity with increasingcurrent density greater for the monoclinic than for the rhombohedralparent phase.

The cathode materials of the invention avoid the phase transition ofknown sulfate cathode materials by incorporating one or more phosphateions as at least one of the constituents of the cathode material. Therhombohedral R3c (NASICON) and monoclinic P2₁/n framework structures ofLi₃Fe₂(PO₄)₃ are similar to those for the sulfates described above, asillustrated in FIG. 5A and FIG. 5B.

A further embodiment of the invention is a rhombohedral NASICON cathodematerial having the formula A_(3−x)V₂(PO₄)₃, where A may be Li, Na or acombination thereof. Rhombohedral A_(3−x)V₂(PO₄)₃ reversiblyintercalates 1.5 Li per formula unit for a discharge capacity of 100mAh·g⁻¹ with average closed-circuit voltage being 3.8 V vs. a lithiumanode (see FIG. 8). The voltage and capacity performances of therhombohedral A_(3−x)V₂(PO₄)₃ compounds of the invention are comparableto the high-voltage cathode materials LiMn₂O₄ (4.0 V), LiCoO₂(4.0 V) andLiNiO₂(4.0 V). The large, open three-dimensional framework of VO₆octahedra and PO₄ tetrahedra allows an easy diffusion of the lithiumions, making it attractive for high-power batteries. A further advantageof this material is that it includes a cheaper and less toxictransition-metal element (V) than the already developed systems usingCo, Ni, or Mn.

EXAMPLES Example 1 Ordered Olivine LiMPO₄ Compounds

The ordered-olivine compound LiFePO₄ was prepared from intimate mixturesof stoichiometric proportions of Li₂CO₃ or LiOH.H₂O, Fe(CH₃CO₂)₂ andNH₄H₂PO₄.H₂O; the mixtures were calcined at 300-350° C. to eliminateNH₃, H₂O, and CO₂ and then heated in Ar at about 800° C. for 24 hours toobtain LiFePO₄. Similar solid-state reactions were used to prepareLiMnPO₄, LiFe_(1−x)Mn_(x)PO₄, LiCoPO₄ and LiNiPO₄. FePO₄ was obtainedfrom LiFePO₄ by chemical extraction of Li from LiFePO₄. Charge/dischargecurves for Li_(1−x)FePO₄ and discharge/charge cycles for Li_(x)FePO₄gave similar results with a voltage of almost 3.5 V vs. lithium for acapacity of 0.6 Li/formula unit at a current density of 0.05 mA·cm²⁻(See FIG. 2A and FIG. 2C). The electrolyte used had a window restrictingvoltages to V<4.3 V. Li extraction was not possible from LiMnPO₄,LiCoPO₄, and LiNiPO₄ with the electrolyte used because these require avoltage V>4.3 V to initiate extraction. However, Li extraction fromLiFe_(1−x)Mn_(x)PO₄ was performed with 0≦x≦0.5, and the Mn³⁺/Mn²⁺ coupleproduced a voltage plateau at 4.0 V vs. lithium.

Arsenate may be substituted for at least some of the phosphate inLiFePO₄. The iron in LiFePO₄ may be supplemented up to 10% by othermetals such as those filled with two electron shells, manganese ortitanium, for example.

This iron-based material is non-toxic, inexpensive, non-hygroscopic,easy to prepare and has good electronic conductivity compared toexisting high voltage cathode materials. There is a decrease in thevolume of the unit cell with the extraction of lithium from LiFePO₄. Athigher current densities, there is a decrease of capacity on repeatedcycling associated with movement of a two-phase interface, a featurecharacteristic of cathodes that traverse a two-phase compositionaldomain in a discharge cycle. The volume change across the two-phaseinterface causes the particles to crack and lose electrical contact withthe current collector. This problem can be overcome by reducing theparticle size of the cathode material to the nanometer scale.

Example 2 Modified Olivine LiMPO₄ Compounds

The modified olivine compounds, LiMPO₄, were prepared from intimatemixtures of stoichiometric proportions of Fe(C₂O₄).2H₂O (Aldrich),LiH₂PO₄, Li₂C₂O₄ (Alpha Inorganics), (NH₄)₂TiO(C₂O₄)₂.H₂O andpolydiethoxy-siloxane (Gelest) in isopropanol. In particular, 12.95grams of iron oxalate (Fe(C₂O₄).2H₂O), 8.31 grams of lithium dihydrogenphosphate (LiH₂PO₄), 1.52 grams of lithium oxalate (Li₂C₂O₄), 2.94 gramsof ammonium titanyl oxalate monohydrate (NH₄)₂TiO(C₂O₄)₂.H₂O) and 2.68grams of poly-diethyoxysiloxane in 120 milliliters (ml) of isopropanolwere ball-milled in a polyethylene container with sintered zirconiaballs for two days. The resulting slurry was evaporated to dryness andtreated in a vacuum furnace at 350° C., the 700° C. under argon (<6 ppmoxygen) to yield a compound with olivine structure of formulaLi_(1.1)Fe_(0.8)Ti_(0.1)P_(0.8)Si_(0.2)O₄.

The compound was delithiated with a solution of bromine in acetonitrileto Li_(0.1)Fe_(0.8)Ti_(0.1)P_(0.8)Si_(0.2)O₄.

A solid-state button type cell was constructed using a 10⁶ molecularweight polyethylene oxide-lithium bis(trifluoromethanesulfonylimide)electrolyte at an oxygen/lithium ratio of 20:1; the positive electrodewas made from a 45/5/50 v/v composite of respectively the olivinematerial, Ketjenblack and an ionic binder of composition similar to thatof the electrolyte, coated on a nickel current collector. The capacitywas 1.2 mAh·cm⁻². The negative electrode was a lithium disc. At 80° C.,85% of the stoichiometric discharge capacity could be obtained andcycled reversibly at 100 mAh·cm⁻².

Example 3 Rhombohedral NASICON Li_(x)M₂(PO₄)₃ Structures

The inventors compared redox energies in isostructural sulfates withphosphates to obtain the magnitude of the change due to the differentinductive effects of sulfur and phosphorus. RhombohedralLi_(1+x)Ti₂(PO₄)₃ has been shown to exhibit a flat open-circuit voltageV_(oc)=2.5 V vs. lithium, which is roughly 0.8 V below the Ti⁴⁺/Ti³⁺level found for FeTi(SO₄)₃. The flat voltage V(x) is indicative of atwo-phase process. A coexistence of rhombohedral and orthorhombic phaseswas found for x=0.5 (Delmas and Nadiri 1988; Wang and Hwu 1992).Li_(2+x)FeTi(PO₄)₃ of the present invention remains single phase ondischarge.

All three phosphates Li₃M₂(PO₄)₃, where M=Fe, Fe/V, or V, have themonoclinic Fe₂(SO₄)₃ structure if prepared by solid-state reaction. Theinventors have found that these compounds exhibit a rhombohedralstructure when prepared by ion exchange in LiNO₃ at 300° C. from thesodium analog Na₃Fe₂(PO₄)₃. The discharge/charge curve of FIG. 6A forlithium insertion into rhombohedral Li_(3+x)Fe₂(PO₄)₃ exhibits anaverage V_(oc) of 2.8 V. This is surprisingly different from the curvesfor the monoclinic form (See FIG. 6B). The inventors have found that upto two lithiums per formula unit can be inserted into Li₃Fe₂(PO₄)₃,leading to Li₅Fe₂(PO₄)₃. The Li⁺-ion distribution in the interstitialspace of Li_(3+x)Fe₂(PO₄)₃, where 0<x<2, appears to vary continuouslywith x with a high degree of disorder. FIG. 7A shows a reversiblecapacity loss on increasing the current density from 0.05 to 0.5mA·cm⁻². A reversible discharge capacity of 95 mAh·g⁻¹ is still observedfor rhombohedral Li_(3+x)Fe₂(PO₄)₃ at a current density of 20 mA·g⁻¹.This is much reduced compared to what is encountered with the monoclinicsystem (See FIG. 7B). With a current density of 23 mA·g⁻¹ (or 1mA·cm⁻²), the initial capacity of 95 mAh·g⁻¹ was maintained in a coincell up to the 40^(th) cycle.

Another cathode material of the present invention, Li₂FeTi(PO₄)₃, havingthe NASICON framework was prepared by solid-state reaction. Thismaterial has a voltage ranging from 3.0 to 2.5 V.

Rhombohedral TiNb(PO₄)₃ can be prepared by solid-state reaction at about1200° C. Up to three Li atoms per formula unit can be inserted, whichallows access to the Nb⁴⁺/Nb³⁺ couple at 1.8 V vs. lithium for x>2 inLi_(x)TiNb(PO₄)₃. Two steps are perhaps discernible in the compositionalrange 0<x<2; one in the range of 0<x<1 corresponds to the Ti⁴⁺/Ti³⁺couple in the voltage range 2.5 V<V<2.7 V and the other for 1<x<2 to theNb⁵⁺/Nb⁴⁺ couple in the range 2.2 V<V<2.5 V. It appears that these redoxenergies overlap. This assignment is based on the fact that theTi⁴⁺/Ti³⁺ couple in Li_(1+x)Ti₂(PO₄)₃ gives a flat plateau at 2.5 V dueto the presence of two phases, rhombohedral LiTi₂(PO₄)₃ and orthorhombicLi₃Ti₂(PO₄)₃. The presence of Nb in the structure suppresses theformation of the second phase in the range 0<x<2.

Rhombohedral LiFeNb(PO₄)₃ and Li₂FeTi(PO₄)₃ can be prepared by ionexchange with molten LiNO₃ at about 300° C. from NaFeNb(PO₄)₃ andNa₂FeTi(PO₄)₃, respectively. Two Li atoms per formula unit can beinserted reversibly into Li_(2+x)FeTi(PO₄)₃ with a little loss ofcapacity at 0.5 mA·cm⁻². Insertion of the first Li atom in the range 2.7V<V<3.0 V corresponds to the Fe³⁺/Fe²⁺ redox couple and of the second Liatom in the range of 2.5 V<V<2.7 V to an overlapping Ti⁴⁺/Ti³⁺ redoxcouple. The insertion of lithium into Li_(1+x)FeNb(PO₄)₃ gives a V vs. xcurve that further verifies the location of the relative positions ofthe Fe³⁺/Fe²⁺, Nb⁵⁺/Nb⁴⁺ redox energies in phosphates withNASICON-related structures. It is possible to insert three lithium atomsinto the structure, and there are three distinct plateaus correspondingto Fe³⁺/Fe²⁺ at 2.8 V, Nb⁵⁺/Nb⁴⁺ at 2.2 V, and Nb⁴⁺/Nb⁵⁺ at 1.7 V vs.lithium in the discharge curve.

The rhombohedral A_(3−x)V₂(PO₄)₃ compounds of the invention can beprepared by ionic exchange from the monoclinic sodium analogNa₃V₂(PO₄)₃. The inventors were also able to prepare the rhombohedralLi₂NaV₂(PO₄)₃ with the NASICON framework by a direct solid-statereaction (FIG. 9). The discharge/charge curves at 0.05 mA·cm⁻² (0.95mA·g⁻¹) for the rhombohedral Li_(x)NaV₂(PO₄)₃ are shown in FIG. 8.

The rhombohedral LiFe₂(SO₄)₂(PO₄) may be prepared by obtaining anaqueous solution comprising FeCl₃, (NH₄)₂SO₄, and LiH₂PO₄, stirring thesolution and evaporating it to dryness, and heating the resulting drymaterial to about 500° C. Discharge/charge curves vs. lithium at 0.1mA·cm⁻² for rhombohedral Li_(1+x)Fe₂(PO₄) (SO₄)₂, where 0<x<3, are shownin FIG. 10.

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. More specifically, it will beapparent that certain agents which are both chemically and structurallyrelated may be substituted for the agents described herein to achievesimilar results. All such substitutions and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope andconcept of the invention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference:

-   Delmas, C., and A. Nadiri, Mater. Res. Bull., 23, 63 (1988);-   Goodenough, J. B., H. Y. P. Hong and J. A. Kafalas, Mater. Res.    Bull. 11, 203, (1976);-   Guyomard, D. and J. M. Tarascon, J. Electrochem. Soc., 139, 937    (1992);-   Long, G. J., G. Longworth, P. Battle, A. K. Cheetham, R. V.    Thundathil and D. Beveridge, Inorg. Chem., 18, 624 (1979);-   Manthiram, A., and J. B. Goodenough, J. Power Sources, 26, 403    (1989);-   Masquelier, C., M. Tabuchi, K. Ado, R. Kanno, Y. Kobayashi, Y.    Maki, O. Nakamura and J. B. Goodenough, J. Solid State Chem., 123,    255 (1996);-   Mizushima, K., P. C. Jones, P. J. Wiseman and J. B. Goodenough,    Mater. Res. Bull., 15, 783 (1980);-   Nanjundaswamy, K. S., et al., “Synthesis, redox potential evaluation    and electrochemical characteristics of NASICON-related 3D framework    compounds,” Solid State Ionics, 92 (1996) 1-10;-   Nishi, Y., H. Azuma and A. Omaru, U.S. Pat. No. 4,959,281, Sep. 25,    1990;-   Okada, S., K. S. Nanjundaswamy, A. Manthiram and J. B. Goodenough,    Proc. 36th Power Sources Conf., Cherry Hill at New Jersey (Jun. 6-9,    1994);-   Schollhorn, R. and A. Payer, Agnew. Chem. (Int. Ed. Engl.), 24, 67    (1985);-   Sinha, S. and D. W. Murphy, Solid State Ionics, 20, 81 (1986);-   Thackeray, M. M. W. I. F. David, J. B. Goodenough and P. Groves,    Mater. Res. Bull., 20, 1137 (1983);-   Thackeray, M. M., P. J. Johnson, L. A. de Piciotto, P. G. Bruce    and J. B. Goodenough, Mater. Res. Bull., 19, 179 (1984);-   Thackeray, M. M., W. I. F. David, P. G. Bruce and J. B. Goodenough,    Mater. Res. Bull. 18, 461 (1983); and Wang, S., and S. J. Hwu, Chem.    of Mater. 4, 589 (1992).

1-61. (canceled)
 62. A synthesized cathode material for a rechargeableelectrochemical cell having two crystalline phases, LiFePO₄ and FePO₄that belong to the same space group and that allow reversible extractionand insertion of lithium ions.
 63. The synthesized cathode material ofclaim 62, wherein the cathode material is porous.
 64. The synthesizedcathode material of claim 62, wherein the cathode material is in theform of particles.
 65. The synthesized cathode material of claim 64,wherein particles have a particle size on a nanometer scale.